FIG. 1 schematically shows prior art image recording apparatus 100 with light-emitting diode (LED) printbar 101. Printbar 101 is an example of an LED full width array imager. An LED full width array imager consists of an arrangement of a large number of closely spaced LEDs in a linear array. By providing relative motion between the LED printbar and a photoreceptor in a process direction, and by selectively energizing the LEDs at the proper times in a scan direction, a desired latent electrostatic image can be produced on the recording member. The production of a desired latent image is usually performed by having each LED expose a corresponding pixel on the recording member in accordance with image-defining video data information applied to the printbar through driver circuitry. Conventionally, digital data signals from a data source, which may be a Raster Input Scanner (RIS), a computer, a word processor or some other source of digitized image data is clocked into a shift register. Some time after the start of a line signal, individual LED drive circuits are then selectively energized to control the on/off timing of currents flowing through the LEDs. The LEDs selectively turn on and off at fixed intervals to form a line exposure pattern on the surface of the photoreceptor. A complete image is formed by successive line exposures.
The following provides further detail regarding prior art apparatus 100. Printbar 101 includes: LED's controlled according to recording signals supplied from an unrepresented external device; a photoreceptor (in the form of a rotary drum) 102 provided with photoconductive layer 119 along the periphery thereof; a rod lens array 103 for focusing the light beams of the LED's in the printing head 101 onto photoconductive layer 119; a corona charger 104 for charging photoconductive layer 119 in advance; a developing station 105 for developing an electrostatic latent image with toner; a recording sheet 106; a cassette 107 housing a plurality of recording sheets 106; a feed roller 108 for feeding the recording sheet 106 from the cassette 107; registration rollers 109 for matching the front end of the recording sheet with the leading end of the image formed on the drum 102; a transfer charger 110 for transferring the developed image from the drum 102 onto the recording sheet 106; a separating roller 111 for separating the recording sheet from the drum 102; a belt 112 for transporting the recording sheet; fixing rollers 113; discharge rollers 114 for discharging the recording sheet onto a tray 115; a blade cleaner 116 for removing the toner remaining on the drum 102; a container 117 for the recovered toner; and a lamp 118 for eliminating charge remaining on the drum 102.
In general, the LEDs in the printbar are operated so as to discharge respective portions of photoconductive layer 119 such that the respective voltage levels for the portions of the photoreceptor are at a first level corresponding to a print area or at a second level corresponding to a blank area. Toner adheres to the photoreceptor according to the charge remaining on the photoconductive layer. Therefore, the first level is relatively high and the second level is relatively low. The operation of the LEDs assumes that the charge on the photoconductive layer, prior to illumination by the LEDs, is at a uniform specified level, in particular, across the width of the photoreceptor (orthogonal to the direction of movement for the photoreceptor). However, the actual charge on various portions of the photoconductive layer can vary from the specified level due to dimensional variances in the photoconductive layer, misalignment of charger 104, or misalignment of drum 102. Such variances in the actual charge levels result in the LEDs discharging the respective portions of the photoconductive layer at levels other than the first or second level noted above, which results in a degradation of print quality for apparatus 100.
For example, the thickness of photoconductive layer 119 is assumed to be at a specified level. However, the actual thickness of the photoconductive layer, in particular, across the width of the photoreceptor, can vary due to fabrication tolerances. The charge carried by the photoconductive is directly related to the thickness of the photoconductive layer. For a particular charging voltage from charger 104, there is an assumed charge on the photoreceptor in accordance with the assumed specified thickness of the photoconductive layer. However, the actual charge carried by the photoreceptor, in response to the uniform charging voltage, can vary according to the actual thickness of the photoconductive layer, which can vary from the specified thickness as noted above. For example, if the actual thickness of the photoconductive layer varies from the assumed thickness such that the charge on a portion of the photoconductive layer is greater than the assumed charge, the LED printbar will not be able to discharge the portion to the first or second level noted above. In this case, the image for the portion may be undesirably darker than surrounding areas.
Regarding the misalignment of charger 104, for example, the distance of the charger from the photoreceptor can vary across the width of the photoreceptor due to the misplacement of the charger or deformations in the structure of the charger. The amount of charging voltage delivered to the photoreceptor depends on the distance between the charger and the photoreceptor. Therefore, the charge on the photoreceptor can vary as the distance between the charger and the photoreceptor varies. Drum misalignment can cause the distance between photoreceptor and the charger to vary along the length of the photoreceptor (along the direction of movement for the photoreceptor). It should be understood that the discussion above is applicable to image recording apparatus with other photoreceptor configurations, such as a photoreceptor belt. For example, misalignment of one or more drums for a photoreceptor belt can cause the drum alignment problems noted above.
U.S. Pat. No. 6,223,006 describes photoreceptor charge control directed toward photoreceptor variation around a periphery of a photoreceptor due to the manufacturing process for the photoreceptor. This patent fails to address variation in areas of the photoreceptor other than along the periphery, for example, variation across a width of the photoreceptor. This patent also fails to address variation due to alignment of the photoreceptor or components acting on the photoreceptor, such as the charger.